Nitric oxide (NO) sensing is a critical capability for a variety of applications ranging from high temperature combustion to clinical analysis. In high temperature combustion applications, detection of nitrogen oxides (NOx) is critical in controlling the processes used to reduce the NOx emissions produced by the leaner combustion processes being developed to improve fuel efficiency. NOx sensors that are high temperature capable may also find use in other high-temperature applications. Another area where NOx sensing is required is in the medical industry, specifically in breath analysis. These do not typically involve applications where the sensor operates in a high temperature ambient environment, but it is one where the detection of nitric oxide (NO) itself has high importance.
There are a variety of ways to detect NO, with solid-state electrochemical sensors being one such technique. Such sensors also have the added benefit of being easier to miniaturize compared to other techniques. A variety of solid-state electrochemical sensors for NO have been demonstrated previously. These techniques vary and a continuing challenge is to design sensitive systems with limited size, weight and power consumption so as to allow for portable sensor systems. Such advancements would have notable impact on the healthcare industry in enabling homecare monitoring units.
NO sensors capable of detecting NO at concentrations as low as 7 ppb have been demonstrated using an array of sensor units in series to increase the resulting sensor signal for a given NO concentration. However, these sensors were made using hand assembly techniques and also were assembled into arrays by hand. This manual fabrication limits the minimum size to which the sensors can be reduced.
Miniaturized sensors based on microelectromechanical systems (MEMS) fabrication technology have been demonstrated for aerospace applications. Sensors made by MEMS fabrication are very small devices that can be made up of components and features between 1 to 100 micrometers in size (0.001 to 0.1 mm). Fabrication is a challenge at these size scales for several reasons. Large surface area to volume ratio of MEMS, and the resulting surface effects which dominate over volume effects can improve sensor performance. However, the overall surface area of a MEMS sensor unit may be notably smaller than corresponding macro sensor devices. This may decrease the overall number of chemical reactions involved, resulting in a decreased signal. Thus, improved sensor design is mandatory to enable miniaturization of sensor systems. Such optimization may be different on the macro level then for micro sensors, and simple application of design principle that are successful for macro sensor can lead to significantly degraded performance for micro sensors.
A reduction in size of the sensors using MEMS techniques would not only decrease the size for better implementation in a handheld home monitoring unit, but the reduced size would also decrease the power required to bring the sensors up to operating temperature. In addition, the utilization of MEMS fabrication techniques introduces batch fabrication that allows for multiple sensors to be made at one time, thus reducing costs.